The velocity of sound in air is doubled when the temperature is raised from $0^{\circ} C$ to $\alpha^{\circ} C$. The value of $\alpha$ is . . . . . . .

The Earth has a radius of $6400 \ km$. The inner core of $1000 \ km$ radius is solid. Outside it,there is a region from $1000 \ km$ to a radius of $3500 \ km$ which is in a molten state. Then again from $3500 \ km$ to $6400 \ km$,the Earth is solid. Only longitudinal $(P)$ waves can travel inside a liquid. Assume that the $P$ wave has a speed of $8 \ kms^{-1}$ in solid parts and of $5 \ kms^{-1}$ in liquid parts of the Earth. An earthquake occurs at some place close to the surface of the Earth. Calculate the time after which it will be recorded in a seismometer at a diametrically opposite point on the Earth if the wave travels along the diameter.

$A$ $SONAR$ (sound navigation and ranging) uses ultrasonic waves to detect and locate objects under water. In a submarine equipped with a $SONAR$,the time delay between generation of a probe wave and the reception of its echo after reflection from an enemy submarine is found to be $77.0\; s$. What is the distance of the enemy submarine (in $; km$)? (Speed of sound in water $= 1450\; m/s$)

When a steel pipe is hammered at one end,we hear two sounds at another end. Why?

Explain the propagation of sound waves in:
$(i)$ air
$(ii)$ solids

Which of the following statements is $NOT$ true?